What is the lowest altitude at certain fixes an aircraft must cross when heading toward a higher minimum en route IFR altitude, depicted with the letter 'X'?

The correct choice is based on the requirement for an aircraft to cross at or above a specified altitude when navigating through a particular point in the airspace where a higher minimum en route IFR altitude is established. This altitude is known as the Minimum Crossing Altitude (MCA).

The MCA ensures that an aircraft maintains safe clearance above terrain and obstacles as it approaches an area where the minimum en route altitude increases due to navigation requirements or terrain considerations. It represents the lowest altitude at which an aircraft may safely cross a specified fix on its route under Instrument Flight Rules (IFR).

In contrast, other options refer to different altitudes associated with navigation and safety. The Minimum En Route Altitude (MEA) is the lowest altitude at which the aircraft can operate safely within the airspace while providing adequate navigational signal reception. The Minimum Reception Altitude (MRA) ensures that an aircraft is above the terrain to receive necessary navigation signals reliably. The Maximum Authorized Altitude (MAA) indicates the maximum altitude at which an aircraft can operate within a given airspace without exceeding the limits of navigational signals. Each of these serves a distinct purpose that differs from the requirement for a specific crossing altitude.